OKLAHOMA CITY (AP) - Tom Coburn’s early retirement from the U.S. Senate boosted interest in this year’s Oklahoma primary by triggering a Republican primary fight for his seat and opening contests further down the ballot.

Only one sitting member of Congress is unopposed in the coming week, and even Gov. Mary Fallin faces challengers in Tuesday’s Republican primary.

Tuesday’s contests are the first tests for political hopefuls who want to see their name on the Nov. 4 general election ballot. Runoff elections, if needed, will be Aug. 26.

Congressman James Lankford and former state House Speaker T.W. Shannon are among those fighting for the GOP nomination for Coburn’s seat, and there are wide open races for Lankford’s central Oklahoma seat.

A quarter of the state’s 2 million voters cast ballots in the 2010 primary. Only registered Democrats and Republicans may vote.

Early voting started Thursday. Among those at the polls then was Democrat Joe Baxter, 57, of Midwest City, who said the democratic process means his vote still counts in a Republican-leaning state.

They can’t “beat the popular vote,” Baxter said.

Baxter said President Barack Obama has been elected twice in spite of the efforts of billionaire opponents.

Topping the ballot are both of Oklahoma’s seats in the U.S. Senate. Senior senator Republican Sen. Jim Inhofe is seeking a fourth six-year term and faces four challengers in the GOP primary. Democrat Matt Silverstein is unopposed for his party’s nomination.

The campaign for Coburn’s open seat attracted the most interest of any statewide election. U.S. Rep. James Lankford and former state House Speaker T.W. Shannon top a slate of seven candidates seeking the Republican nomination that also includes former state Sen. Randy Brogdon and four others.

Three candidates are seeking the Democratic nomination for the seat: state Sen. Connie Johnson, Patrick Michael Hayes and Jim Rogers.

The race for Lankford’s open 5th Congressional District seat in central Oklahoma is the most competitive of Oklahoma’s five seats in the U.S. House. Six Republicans - Corporation Commissioner Patrice Douglas, former state Rep. Shane Jett, state Rep. Mike Turner, state Sen. Clark Jolley, former state Sen. Steve Russell and the Rev. Harvey Sparks - are seeking the GOP nomination.

Three Democrats are seeking their party’s nomination in the primary: state Sen. Al McAffrey, retired university professor Tom Guild and seniors advocate Leona Leonard.

First-term Republican Rep. Markwayne Mullin of the 2nd District faces professional fisherman Darrel Robertson for the GOP nomination for the sprawling eastern Oklahoma district. Two Democrats, Earl E. Everett and Joshua Harris-Till, are seeking their party’s nomination in the primary.

In western Oklahoma’s 3rd District, 10-term Rep. Frank Lucas faces two challengers for the Republican nomination. Democrat Frankie Robbins is unopposed for his party’s nomination.

In the 4th District, six-term Rep. Tom Cole has one opponent for the Republican nomination. Two Democrats are seeking the Democratic nomination.

Republican Rep. Jim Bridenstine was unopposed for re-election in the 1st District.

Fallin, seeking a second term as Oklahoma’s chief executive, faces two challengers for the Republican nomination. State Rep. Joe Dorman is unopposed for the Democratic nomination.

Republican Superintendent of Education Janet Barresi faces two challengers for the GOP nomination, including former state Board of Education member Joy Hofmeister. Four candidates are seeking the Democratic nomination.

Primary elections are also scheduled in more than 40 of the 149 seats in the Oklahoma House and Senate.